Wireless communication systems are well known in the art. Communications standards are developed in order to provide global connectivity for wireless systems and to achieve performance goals in terms of, for example, throughput, latency and coverage. One current standard in widespread use, called Universal Mobile Telecommunications Systems (UMTS), was developed as part of Third Generation (3G) Radio Systems, and is maintained by the Third Generation Partnership Project (3GPP).
A typical UMTS system architecture in accordance with current 3GPP specifications is depicted in FIG. 1. The UMTS network architecture includes a Core Network (CN) interconnected with a UMTS Terrestrial Radio Access Network (UTRAN) via an Iu interface. The UTRAN is configured to provide wireless telecommunication services to users through wireless transmit receive units (WTRUs), referred to as user equipments (UEs) in the 3GPP standard, via a Uu radio interface. The UTRAN may have one or more radio network controllers (RNCs) and base stations, referred to as Node Bs by 3GPP, which collectively provide for the geographic coverage for wireless communications with UEs. One or more Node Bs may be connected to each RNC via an Iub interface. RNCs within a UTRAN communicate via an Iur interface.
One type of air interface defined in the UMTS standard is wideband code division multiple access (W-CDMA). In a W-CDMA system baseband signals are spread in the frequency domain using orthogonal spreading codes prior to transmission, and despread at a receiver using the same spreading codes.
The Uu radio interface of a 3GPP system uses transport channels (TrCHs) for transfer of user data and signaling between UEs and Node Bs. Uplink refers to signaling from a UE to a Node B, and downlink transmissions are from a Node B to a UE. In 3GPP communications, TrCH data is conveyed by one or more physical channels defined by mutually exclusive physical resources, or shared physical resources in the case of shared channels. In a conventional 3GPP system, communications between a UE and a Node B are conducted using a single data stream defined by a combination of TrCHs called a coded composite TrCH (CCTrCH). Typically, a Node B is concurrently communicating with several UEs using respective CCTrCH data streams.
TrCH data is transferred in sequential groups of transport blocks (TBs) defined as transport block sets (TBSs). Each TBS is transmitted in a given transmission time interval (TTI) which may span a plurality of consecutive system time frames. The number of bits in a TBS is called the transport block set size (TBSS).
UMTS specification releases 5 and 6 pertain to high speed downlink packet access (HSDPA) and high speed uplink packet access (HSUPA), respectively. HSDPA is a downlink packet access protocol for packet based UMTS wireless communication systems employing a W-CDMA air interface with a spreading factor (SF) of 16. According to HSDPA, up to 15 spreading codes may be allocated to data for transmission in a common TTI. The data may be modulated using either quadrature phase shift keying (QPSK) modulation or 16 quadrature amplitude modulation (16-QAM). In future releases of the HSDPA standard, it is expected that additional types of higher order modulation will also be supported, such as 64 quadrature amplitude modulation (64-QAM). Fast retransmissions are accomplished according to hybrid automatic repeat request (HARQ) by retransmission combining, which enables operation at relatively high Block Error Rates (BLER).
A the CQI mapping table, as in Table 1 for example, indicates a preferred TFRC for a given CQI according to conventional TFRC selection approaches for HSDPA, the TFRC parameters of TBSS, number of spreading codes, and modulation are mutually dependent. Therefore, multiple different TFRCs may be able to match the desired channel characteristics corresponding to a given CQI level, including maximum expected data rate and TB success probability.
TABLE 1CQI mapping table for UE category10 according to 3GPP TS 125.214.Number of spreadingCQITBSScodesModulation0N/AOut of range11371QPSK21731QPSK32331QPSK43171QPSK53771QPSK64611QPSK76501QPSK87922QPSK99312QPSK1012622QPSK1114833QPSK1217423QPSK1322793QPSK1425834QPSK1533194QPSK163565516-QAM174189516-QAM184664516-QAM195287516-QAM205887516-QAM216554516-QAM227168516-QAM239719716-QAM2411418816-QAM25144111016-QAM26172371216-QAM27217541516-QAM28233701516-QAM29242221516-QAM30255581516-QAM
Conventional strategies for selecting a TFRC include choosing a fewer number of spreading codes N than the maximum number of available spreading codes M because the total allocated power Pt is divided among the N used spreading codes. Therefore it is believed that received signal quality is better when more power is allocated per spreading code.
The inventor has recognized, however, that higher power for each spreading code increases interference with the other spreading codes in the channel, and employing fewer spreading codes does not necessarily provide better performance results, particularly in receivers employing advanced decoding techniques. Existing TFRC selection procedures do not take into account the interference effects of simultaneous transmissions using different spreading codes or the capabilities of advanced receivers. Therefore, a procedure for TFRC selection that improves upon the existing techniques is desired.